DESCRIPTION (Applicant's abstract reproduced verbatim): The hedgehog (Hh) family of secreted proteins plays a fundamental role in cell differentiation within the brain, spinal cord, limbs, somites and circulatory system. Defects in Hh signaling during embryonic development are associated with human congenital malformations, including holoprosencephaly. Mis-regulation of Hh signaling later in life is associated with basal cell carcinoma, the most common form of cancer affecting fair-skinned adults. The study of Hh signaling during development is thus important for understanding human congenital malformations as well as the formation of certain tumors. The major goal of this project is to understand how Hh signaling contributes to cell differentiation during normal development in the zebrafish embryo. We have used a genetic approach to study the role of Hh signaling in the formation of a defined region of the nervous system, the post optic area of the ventral forebrain. We have recently shown that two zebrafish forebrain mutations encode Hh-responsive transcription factors of the gli family. Cell differentiation defects in the zebrafish forebrain mutants appear to be caused by mis-regulation of Hh signaling. Consistently, it was also recently demonstrated that another zebrafish forebrain mutation encodes sonic hedgehog. The fact that three of the zebrafish forebrain mutants encode members of the Hh pathway underscores the importance of this signaling cascade in development. This application focuses on a fourth zebrafish forebrain mutant called umleitung (uml). Like the known Hh pathway mutations, uml interferes with Hh signaling and cell differentiation in the ventral forebrain. We now have evidence that uml may encode another zebrafish gli gene. While gli genes appear to be central to the regulation of Hh signaling, little is known about how gli genes function in vertebrates. There is evidence that some gli genes activate the transcription of Hh target genes, while others act to repress Hh targets. We propose to analyze the role of gli mediated hedgehog signaling in zebrafish forebrain development by A) identifying the gene encoded by uml, determining the genetic lesion in uml, and isolating null alleles of uml, B) determining how gli genes contribute to cell differentiation in the ventral forebrain, C) determining how gli genes regulate, and in turn are regulated by, Hh signaling in vivo, and D) identifying and characterizing genes that are transcriptionally regulated by gli genes in the zebrafish embryo. The zebrafish embryo provides a powerful model for the study of Hh regulation in vertebrates as it allows a genetic approach to be combined with ectopic expression studies. The combination of these two approaches is well suited to the study of molecular signaling pathways that regulate cell fate decisions and promises to provide insights not readily attainable in other vertebrate systems.